program Project2;

{$APPTYPE CONSOLE}

uses
  SysUtils;
var
 n,a,i, sum: integer;

begin
  sum:=0;
  readln(n);
  if (n mod 2 =0) then
    for i:=0 to (n div 2) do
      sum:=sum+ i*2;
  if (n mod 2 <>0) then
  begin
    sum:=-1;
    for i:=0 to ((n-1) div 2) do
      sum:=sum+ i*2 + 1;
  end;
  writeln(sum);
  readln;
  readln;

end.
